A radio system generally includes a transmitter that transmits information-carrying signals to a receiver. The transmitter includes a power amplifier that operates to amplify the signal to be transmitted to a power level that is sufficient to enable receipt of the signal by the receiver. Radio system transmitters are required to satisfy specifications for signal levels at frequencies other than the intended transmission frequencies. Some specifications are set by government regulatory bodies, while others are set by radio communications standards such as 3GPP or IEEE 802.11. One specification, or requirement, is adjacent channel power, which is directly related to power amplifier linearity. Power amplifier linearity corresponds to an ability to reproduce an amplified version of the input signal. Also, power amplifiers are often described in terms of their efficiency, which is defined as some comparison between average transmit signal power and total average power required to generate the transmit signal power.
At a circuit level, power amplifier linearity may be achieved by biasing transistors in such a manner that the power amplifier operates in a linear fashion. However, doing so has a cost in terms of very low operating efficiency. As such, many modern power amplifiers are configured to operate at maximum efficiency, resulting in poor linearity, and use so-called “linearization” circuitry to correct non-linearity. Some exemplary power amplifiers that have high efficiency, but low linearity, are Class AB power amplifiers, Class B power amplifiers, Class C power amplifiers, Class F power amplifiers, Doherty power amplifiers, and Chireix power amplifiers.
Various linearization schemes have evolved having various trade-offs in terms of linearity, power dissipation, and versatility or robustness. These linearization schemes include, but are not limited to, analog predistortion, digital predistortion, feed-forward linearization, and feedback linearization. Predistortion linearization uses a predefined model of power amplifier non-linearity to generate an “opposite” nonlinear response that compensates for the non-linearity of the power amplifier. By amplifying the predistorted signal, the output of the power amplifier is as if the power amplifier were linear.
More specifically, FIG. 1 illustrates a conventional transmitter 10 without predistortion or, for that matter, any other linearization technology. As illustrated, the transmitter 10 includes a modem 12, an up-converter 14, a power amplifier (PA) 16, and a filter 18 connected as shown. The modem 12 outputs a baseband signal (SBB) to the up-converter 14. The up-converter 14 operates to up-convert the baseband signal (SBB) to a desired radio frequency, which is referred to as a carrier frequency (fC), to thereby provide a radio frequency signal (SRF). The power amplifier 16 then amplifies the radio frequency signal (SRF) to a desired output power level to output an amplified radio frequency signal (SRF—AMP). Notably, as discussed below, the amplified radio frequency signal (SRF—AMP) contains distortion due to a non-linearity of the power amplifier 16. The amplified radio frequency signal (SRF—AMP) is then filtered by the filter 18 to remove out-of-band frequency components to thereby provide an output signal (SOUT) to be transmitted by the transmitter 10.
FIGS. 2A through 2D are frequency band diagrams for the various signals in the transmitter 10 of FIG. 1. Specifically, FIG. 2A is a frequency band diagram for the baseband signal (SBB). As shown, the baseband signal (SBB) is centered at DC and has been sampled at a baseband sampling rate (fS—BB). FIG. 2B is a frequency band diagram for the radio frequency signal (SRF) resulting from the up-conversion of the baseband signal (SBB) to the desired carrier frequency (fC). Importantly, FIG. 2C is a frequency band diagram for the amplified radio frequency signal (SRF—AMP) output by the power amplifier 16. When compared to the frequency band diagram for the radio frequency signal (SRF) prior to amplification as shown in FIG. 2B, the frequency band diagram of FIG. 2C clearly illustrates a frequency-spreading effect resulting from distortion caused by the non-linearity of the power amplifier 16. Lastly, FIG. 2D is a frequency band diagram for the output signal (SOUT) output by the filter 18.
FIG. 3 illustrates a conventional transmitter 20 that performs predistortion to compensate for distortion caused by power amplifier non-linearity. As illustrated, the transmitter 20 includes a modem 22, up-sampling circuitry 24, a predistorter (PD) 26, an up-converter 28, a power amplifier (PA) 30, and a filter 32 forming a forward path of the transmitter 20. The modem 22 outputs a baseband signal (SBB) to the up-sampling circuitry 24. The up-sampling circuitry 24 up-samples the baseband signal (SBB) to a predefined sampling rate for predistortion to thereby provide an up-sampled baseband signal (SBB—US). As discussed below, the sampling rate is greater than a bandwidth of a predistorted signal (SPD) output by the predistorter 26. The predistorter 26 predistorts the up-sampled baseband signal (SBB—US) based on a defined predistortion characteristic (e.g., an N-th order polynomial predistortion characteristic) to provide the predistorted signal (SPD). The predistortion applied by the predistorter 26 compensates for (e.g., cancels or substantially cancels) a distortion resulting from a non-linearity of the power amplifier 30. The up-converter 28 upconverts the predistorted signal (SPD) to a desired carrier frequency to provide a radio frequency signal (SRF), which is then amplified to a desired output power level by the power amplifier 30 to provide an amplified radio frequency signal (SRF—AMP). As a result of the predistortion applied by the predistorter 26, the amplified radio frequency signal (SRF—AMP) is as if the power amplifier 30 was a linear power amplifier. The filter 32 then removes any residual out-of-band distortion from the amplified radio frequency signal (SRF—AMP) to provide an output signal (SOUT) that is transmitted by the transmitter 20.
In order to dynamically configure the predistorter 26, the transmitter 20 also includes a feedback path including a filter 34, an attenuator 36, a down-converter 38, and an adaptor 40 connected as shown. The filter 34 is coupled to the output of the power amplifier 30 and operates to remove out-of-band frequency components from the amplified radio frequency signal (SRF—AMP) to provide a radio frequency feedback signal (SFB—RF). The attenuator 36 then attenuates the radio frequency feedback signal (SFB—RF) by a factor 1/G, where G is equal to or approximately equal to a gain of the power amplifier 30, to thereby provide an attenuated radio frequency feedback signal (SFB—RF—1/G). The down-converter 38 then down-converts the attenuated radio frequency feedback signal (SFB—RF—1/G) to baseband to provide a baseband feedback signal (SFB—BB). Based on the baseband feedback signal (SFB—BB) and the up-sampled baseband signal (SBB—US), the adaptor 40 dynamically configures the predistorter 26 using a known adaptation technique.
FIGS. 4A through 4E are frequency band diagrams for the various signals in the forward path of the transmitter 20 of FIG. 3. Specifically, FIG. 4A is a frequency band diagram for the baseband signal (SBB), which has a sampling rate (fS—BB). FIG. 4B is a frequency band diagram for the up-sampled baseband signal (SBB—US) output by the up-sampling circuitry 24, which has a sampling rate (fS—PD), where fS—PD>fS—BB. Next, FIG. 4C is a frequency band diagram for the predistorted signal (SPD) output by the predistorter 26. As shown, a frequency spreading effect results from the predistortion applied by the predistorter 26. The sampling rate (fS—PD) is selected such that fS—PD/2 is greater than ½ of a bandwidth of the predistorted signal (SPD). FIG. 4D is a frequency band diagram for the radio frequency signal (SRF) output by the up-converter 28. The radio frequency signal (SRF) is centered as a desired carrier frequency (fC). Lastly, FIG. 4E is a frequency band diagram for the amplified radio frequency signal (SRF—AMP) output by the power amplifier 30. As shown, the predistortion applied by the predistorter 26 compensates for the non-linearity of the power amplifier 30 such that the amplified radio frequency signal (SRF—AMP) is as if the power amplifier 30 were a linear power amplifier.
FIGS. 5A through 5C are frequency band diagrams for the various signals in the feedback path of the transmitter 20 of FIG. 3. Specifically, FIG. 5A is a frequency band diagram for the radio frequency feedback signal (SFB—RF) output by the filter 34. FIG. 5B is a frequency band diagram for the attenuated radio frequency feedback signal (SFB—RF—1/G) output by the attenuator 36. Lastly, FIG. 5C is a frequency band diagram for the baseband feedback signal (SFB—BB) output by the down-converter 38.
In the transmitter 20 of FIG. 3, predistortion is performed for a single band signal. However, many modern applications use dual-band signals. As used herein, a dual-band signal is a signal that occupies two distinct frequency bands. More specifically, a dual-band signal contains frequency components occupying a certain continuous bandwidth referred to as a first frequency band and frequency components occupying another continuous bandwidth referred to as a second frequency band. The dual-band signal contains no frequency components between the first and second frequency bands. One exemplary application for dual-band signals is a multi-standard cellular communication system. A base station in a multi-standard cellular communication system may be required to simultaneously, or concurrently, transmit signals for two different cellular communications protocols (i.e., transmit a dual-band signal). Similarly, in some scenarios, a base station in a Long Term Evolution (LTE) cellular communications protocol may be required to simultaneously transmit signals in separate frequency bands.
FIG. 6 illustrates a conventional dual-band transmitter 42. The dual-band transmitter 42 includes a first modem 44 that outputs a first baseband signal (SBB1) and a first up-converter 46 that up-converts the first baseband signal (SBB1) to a first carrier frequency (fC1) to thereby provide a first radio frequency signal (SRF1). The dual-band transmitter 42 also includes a second modem 48 that outputs a second baseband signal (SBB2) and a second up-converter 50 that up-converts the second baseband signal (SBB2) to a second carrier frequency (fC2) to thereby provide a second radio frequency signal (SRF2). A combiner 52 combines the first and second radio frequency signals (SRF1 and SRF2) to provide a combined radio frequency signal (SRF—COMB), which is a dual-band signal. A power amplifier (PA) 54 then amplifies the combined radio frequency signal (SRF—COMB) to a desired output power level to thereby provide an amplified radio frequency signal (SRF—AMP), which is also a dual-band signal. A filter 56 then removes out-of-band, or undesired, frequency components from the amplified radio frequency signal (SRF—AMP) to provide an output signal (SOUT).
FIGS. 7A through 7G are frequency band diagrams for the various signals in the dual-band transmitter 42 of FIG. 6. Specifically, FIG. 7A is a frequency band diagram for the first baseband signal (SBB1), where the sampling rate for the first baseband signal (SBB1) is fS—BB. FIG. 7B is a frequency band diagram for the first radio frequency signal (SRF1) output by the first up-converter 46. Likewise, FIGS. 7C and 7D are frequency band diagrams for the second baseband signal (SBB2) and the second radio frequency signal (SRF2), respectively. FIG. 7E is a frequency band diagram for the combined radio frequency signal (SRF—COMB) output by the combiner 52. As illustrated, the combined radio frequency signal (SRF—COMB) is a dual-band signal having a first frequency band centered at the first carrier frequency (fC1) and a second frequency band centered at the second carrier frequency (fC2).
FIG. 7F is a frequency band diagram for the amplified radio frequency signal (SRF—AMP) output by the power amplifier 54. As a result of the non-linearity of the power amplifier 54, a frequency-spreading effect is seen for the frequency bands centered at the first and second carrier frequencies (fC1 and fC2). In addition, as a result of third-order intermodulation distortion caused by the non-linearity of the power amplifier 54 and the dual-band nature of the combined radio frequency signal (SRF—COMB) input to the power amplifier 54, the amplified radio frequency signal (SRF—AMP) also includes frequency bands centered at frequencies of 2fC1-fC2 and 2fC2-fC1. Note that, while not shown, the combined radio frequency signal (SRF—COMB) may also include higher order intermodulation distortion. Lastly, FIG. 7G is a frequency band diagram for the output signal (SouT), which shows that the filter 56 removed the unwanted frequency bands (e.g., the frequency bands resulting from the third-order intermodulation distortion centered at the frequencies of 2fC1-fC2 and 2fC2-fC1).
Predistortion to compensate for power amplifier non-linearities for a dual-band transmitter presents several problems. This is particularly true if the dual-band transmitter is desired to include a predistorter that simultaneously performs predistortion for each of the frequency bands of the dual-band signal. As such, there is a need for systems and methods for performing predistortion in a dual-band transmitter.